The present invention relates to a method for performing gear shifting by engaging a coupling element with a gear wheel, comprising the step of moving the coupling element relative to the gear wheel from a disengaged position to an engaged position, in which a particular gear is selected. The coupling element normally forms an engagement sleeve.
In one known type of transmission, the gear wheel is axially locked and freely rotatably mounted on a transmission shaft when it is not used to deliver engine power to the transmission shaft. The gear wheel is further in mesh with a secondary gear wheel mounted on a further shaft and is rotated by the further shaft. The engagement sleeve, mounted on the same transmission shaft as the gear wheel, is rotationally fixed by means of splines. The engagement sleeve is therefore movable in the axial direction of the transmission shaft. The gear wheel and the engagement sleeve are provided with further splines for engagement with each other. Thus, the gear wheel can be locked to the transmission shaft with the aid of the engagement sleeve in order to transmit power to the transmission shaft.
Further, according to prior art, the rotation of the gear wheel is synchronized with the rotation of the transmission shaft prior to the engagement sleeve is moved into engagement with the gear wheel. However, shift rejection (gear clashing) often take place during movement of the coupling sleeve as there is not always precise alignment of the splines of the gear wheel and the splines of the coupling sleeve. This may lead to a delayed shifting as the coupling sleeve and the gear wheel may move in unison due to friction between opposite surfaces of the coupling sleeve and the gear wheel.
The invention is particularly related to gear shifting in a vehicle transmission. A vehicle that is powered by an internal combustion engine requires a transmission for shifting gears to effectively transmit the power generated by the engine to the wheels.
Work vehicles are designed to perform relatively short work cycles and therefore change gears frequently. Work vehicles therefore particularly require fast and reliable gear shifting. The term work vehicle comprises different types of material handling vehicles like construction machines, such as a wheel loader, an articulated hauler, a backhoe loader and a motor grader. The invention will be described below in a case in which it is applied in a wheel loader.
The work cycles for a wheel loader may comprise a transportation cycle (>500 m), a load carrying cycle (75-500 m), a close handling cycle (15-75 m) and a short-cycle loading (0-15 m).
During short-cycle loading of for example gravel, the wheel loader is forwarded into a heap of gravel while filling the bucket. The wheel loader is thereafter reversed and turned, driven forwards again and the gravel is unloaded on a container of an articulated hauler or truck. The wheel loader is thereafter reversed and turned again, while returning the bucket to a dig position for repeating the work cycle. Since gears will be shifted frequently, there is a particular need for fast and reliable gear shifting in order to achieve a fast and smooth operation of the wheel loader.
However, the invention is not limited for use in a work vehicle and specifically not to vehicles which repeatedly perform relatively short work cycles. Thus, the invention may for example be applied in other heavy duty vehicles like trucks or buses, or passenger cars.
It is desirable to achieve a faster and more reliable gear shifting and more specifically to guarantee a maximum time for each gear shifting operation. It is particularly desirable to solve the problem of delayed gear shifting due to gear rejection when there is an initial clash/collision between the engagement sleeve and the gear wheel during the gear shifting.
According to an aspect of the present invention, a method comprises moving the coupling element relative to the gear wheel from a disengaged position to an engaged position, in which a particular gear is selected, characterized in that a speed of the coupling element relative to a speed of the gear wheel is controlled during movement of the coupling element so that a speed difference is maintained until the coupling element reaches the engaged position. The term “speed” refers to rotational speed. Thus, the force enabling the axial movement of the coupling element (sleeve) is maintained until engagement is reached.
During operation of the vehicle and before initiation of the gear shifting operation, there is a certain speed difference between the coupling element and the gear wheel. When the gear shifting operation is initiated, a predetermined speed difference is established. The wording “a speed difference is maintained” should not be regarded as limited to that the same speed difference is maintained but instead comprise that the speed difference is changed. Thus, the same speed difference is not necessarily maintained, preferably, there is at no point the same speed.
According to a preferred embodiment, the method involves continuously controlling the speed difference so that it is maintained irrespective of any collision (shift rejection) between engagement means of the coupling element and the gear wheel until the coupling element reaches the engaged position. Thus, the sleeve is continuously forced to move in a circumferential direction relative to the gear wheel during the complete axial movement to engagement independent from any shift rejections during the movement. In other words, the force effecting the sleeve is substantially larger than any counter force arising from such a collision. The engagement means may for example comprise splines or other toothed structure.
According to a preferred embodiment, the method involves ceasing the controlled speed difference between the coupling element and the gear wheel when the engagement position is reached. Preferably, the maintained speed difference is transmitted from the coupling element and the gear wheel to a synchronizer unit when the engagement position is reached. The synchronizer unit preferably comprises two engaging force transmitting elements and the speed difference is absorbed by permitting slippage between the force transmitting elements. The engaging force transmitting elements may be formed by friction discs.
According to a preferred embodiment, the method involves sensing the relative position of the coupling element and the gear wheel for detecting when the engagement position is reached.
Further preferred embodiments of the invention and advantages connected thereto will be apparent from the following drawings, and description.